The present invention relates to a method of measuring the dimensions of microstructured patterns by use of a scanning electron microscope, and to the scanning electron microscope to be used for the above measurement; the invention relates more particularly to a method in which the dimensions of the samples varying in shape are to be measured by radiating electron beams, and to the scanning electron microscope to be used for the above measurement.
During the manufacture and inspection of semiconductor devices, thin-film magnetic heads, and other functional element products by use of microstructured surface processing, scanning electron microscopes are commonly used to measure the widths of processed patterns (hereinafter, this measurement process is referred to as length measurement) and to visually inspect the patterns.
The conventional scanning type of electron microscope is an apparatus intended to form images of samples, wherein the electron beam that has been emitted from an electron source and dimensionally restricted by a convergent lens/object lens combination which utilizes the mutual action between a magnetic field or an electric field and the electron beam is applied to the sample for its one-dimensional or two-dimensional scanning by use of a deflector, then the secondary signals (secondary electron, reflected electron, and electromagnetic wave) that have been generated from the sample by the irradiation of the electron beam are detected using a detector which utilizes a photo-electric effect or the like, and the detected signals are converted and processed into visible signals such as luminance signals synchronized with electron beam scanning of the sample (hereinafter, these signals are referred to as “image signals”).
For the conventional scanning type of electron microscope, efforts are exerted so that the image corresponding to the shape of the sample to be observed and measured in length can be obtained with high accuracy. That is to say, when the surface of a sample is observed, conversion/processing into image signals takes place in a plane area accurately analogous to the corresponding scan area (hereinafter, the plane area is referred to as the image area), and the image signals from the various points in the scan area are also arranged at positions accurately analogous to those of the scan area. This arrangement can usually be implemented by:
1) Making both the scan area and the image area rectangular and constituting one side of each rectangle as length with the same number of scanning lines, and
2) Matching the scan area and the image area in terms of the ratio between the scanning line length and the scanning line-to-line distance.
Thus, the distance between any two points on the sample surface always has a constant ratio with respect to the distance between the corresponding two points on the sample image. This ratio is the magnification of the scanning electron microscope. Such an art has already been commonly realized as the basic technology for constructing a scanning electron microscope, and this art is described in, for example, on pages 2 onward of “SCANNING ELECTRON MICROSCOPY”, a writing by L. Reimer, a German scientist.
In addition, the distance between any two points on the sample surface can be easily calculated from the thus-obtained sample image. This calculation is generally called “length measurement”, and a scanning electron microscope having the relevant calculating function is called the “length-measuring electron microscope.”
Japanese Application Patent Laid-Open Publication No. 2001-147112, on the other hand, describes an example in which the scan area on the sample surface and the sample image are not analogous. In this example, in order to dimensionally measure the patterns of a sample that absolutely require reduction in magnification because each pattern is spaced in spite of consisting of very small elements, an image of the sample is prolonged in a vertical direction with respect to the straight line connecting any two points on the sample. Thus, a secondary electron image is formed for improved dimensional measuring accuracy.